Seats in aircraft have been slidably mounted on tracks and held down by means of fittings. Some fittings can be inserted through the base of the seat leg, screwed into the track, and held in place by a lock nut. Other fittings have heads and a spring biased locking plunger which extends into the track when released. A track mounting is believed to be advantageous for railway cars, but known fittings have not been entirely satisfactory. For example, the above mentioned fittings would rattle when subjected to the severe vibrations which occur in a railway car in normal use. A rattleproof fixture has been proposed, for example in U.S. Pat. 3,620,171, but is believed to be unduly complicated.